Digital
Television
A digital TV needs to derive five video
voltages from a video source to create a picture:
1. Red
2. Green3.Blue
4.
Horizontal Sync
5. Vertical
Sync
(H
& V are the synchronizing start and end points of each
frame.)

A digital video source feeds
component video[(Y) (R-Y)
(B-Y)]* to a television video processor. The
television decoding processor converts component video to RGB and
builds each picture frame progressively line by line. The horizontal and vertical syncronizing data is decoded from additional encoded data.

*Handbook Note:Refer to NTSC Television for an explanation of component video.

OLED,
LCD, and plasma TV processors receive the progressive data but hold it until the
processor has a complete frame of lines. They then flash the
entire frame to the
screen. DLP projectorsbuild
each frame progressively
line
by line on the screen.

MPEGVideo CompressionMPEG video compression is a cycle of incomplete compressed video frames inserted between complete uncompressed frames. Compression eliminates redundant video frame data. This is
somewhat similar to
animation. Unchanging elements of a moving scene,
such as
a static background of blue sky, are held and repeated frame
after frame. Changing data of motion and color passes without (or with less) interuption.
The amount of data reduction plus the number
of incomplete
frames placed between complete frames is determined by the version of MPEG used: MPEG 1, MPEG
2, MPEG 4, or MPEG HEVC. Each
succeeding version increases the amount data reduction needed to
handle the bandwidth needs of each broadcast format. HDTV
off-air broadcast employs MPEG2, Satellite TV broadcast uses
MPEG4, UltraHDTV ATSC 3.0 broadcast will use MPEG HEVC.

ATSC 8VSB
The
final stage of an off-air digital ATSC TV broadcast employs ATSC 8VSB
RF
modulation. 8VSB divides MPEG video and audio into
interleaving packets of
data called PIDS. It is a continuing cycle of a video
PID followed by an audio PID. An ATSC 8VSB TV tuner receives, decodes, and restores the order of the data.

QAM Digital cable
TV tuners use QAM RF
modulation. QAM is a variation of the previous interweaving of the audio
and
video data PID method.

Satellite TV Distribution
TV programming is distributed
via an
umbrella of communication
satellites to local broadcasters, cable
companies, and “small dish” satellite TV providers. Several
dozen
communication satellites
hover in geo-synchronous orbit more than
22,000 miles above the equator. They receive and redirect
programming
to your TV provider’s “tree farm” of large dish satellite
antennas. Local
broadcasters retransmit the programming via a broadcast tower to
roof top antennas. Cable TV providers redistribute programming
via
cable wired directly to residences. Small dish satellite
providers up-link their programming to proprietary satellites which bounce the
programming back to small roof top satellite dish antennas. A small number
of viewers receive their programming directly from the same
satellites as the TV providers. This requires a large 8 to 10
foot diameter dish antenna that must be aimed at the intended
satellite.

Dish antenna, why ?
A local TV
broadcaster may engage thousands of watts of broadcast power. As a
result, the local TV receiving antenna can be as little as a “rabbit
ear” antenna sitting on the TV. In contrast, satellites are limited to only five to fifteen
watts of down-link broadcast power.In
addition, they broadcast
at radio frequencies that are impractical for an electronic tuner
to manage. Consequently the satellite receiving system,
small or large dish, needs a major
assist.Assistance
comes in the form of the satellite dish and a low
noise block feedhorn (LNBF) mounted in front of the dish. The
dish
magnifies the
incoming signal hundreds of fold by reflecting and focusing the
energy at a mounting arm that holds the actual antenna element located in the LNBF.
The LNBF
amplifies the signal still further and
shifts the incoming signal from the 4GHz or 12GHz region to
the more manageable neighborhood of 950 MHz to 1450 MHz.In
addition, small dish operators (DishNet / DirectTV) employ MPEG4 digital
compression to squeeze hundreds of channels within
their bandwidth.
The combination of the dish
antenna, the
LNBF, and MPEG 4 produce a signal that a satellite receiver can easily
handle and display on a TV.
Digital TV Result
RGBHV (derived
from a TV camera prismatic lens and CCD sensors) is converted to component
video.
Broacasters digitally encode the video and compress its data.
A digital TV tuner and video processor restores the broadcasted
data to
RGBHV and recreates the original sequence of pixel framed
images on the TV screen.